If you want to stick to a gear tooth sensor, you might look into anti-lock
brake sensors.

Some front sensors on FWD cars are designed to be exposed to road debris
(hardened stainless steel enclosure) and rear sensors on trucks and SUVs
actually penetrate the differential housing (sensor exposed to axle grease).
New replacement parts are probably more expensive than you would be
interested in ($20 -$100 depending on make / model / application). But I
expect you could pick up some used ones from a scrap yard for next to
nothing.

-Denny

----- Original Message -----
From: "Win Wiencke" <Win@SLYCURVES.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, September 13, 2003 12:11 PM
Subject: Re: [EE]:Shaft position indicator


> Try attaching a small rare earth magnet to the back of the (UGN 3503U) to
> create a field.  Now when the sprocket tooth passes nearby you'll be
> measuring a much more significant effect.  You can get fancy by making a
> steel "U" so the tooth acts like a pole piece.
>
> That said, the gap between the tooth and the Hall detector still has to be
> small (an unscientific wild a** guess is no more than 0.1") and the magnet
> will pick up bits of metal as the gears wear over time.  The grease,
> however, should be no problem except as a chemical insult to whatever
> potting material you encapsulate the detector in.
>
> BTW I seem to recall that there are quite a few Hall detector assemblies
> (and proximity detectors) already sealed in convenient stainless steel
> studs.  Alas, I don't have any current part numbers to offer you because
> we've been "rolling our own" on contact lens lathes for years now.
>
> Cheers,
>
> Win Wiencke
>
> > I have a need to know the position of a shaft, within one degree
accuracy,
> > that rotates at a speed of one revolution in two minutes. The shaft can
> only
> > make one turn.
> >
> > I have been experimenting with a Hall Effect device (UGN 3503U) trying
to
> > count the teeth on the sprocket that turns the shaft. Although
promising,
> I
> > only get a one millivolt difference between the tooth and the gap and
the
> > Hall Effect device has to almost touch the sprocket.<snip>
>
> --
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